The present invention relates to a print head and, more particularly, to a printing mechanism of a print head in a dot matrix printer.
Dot matrix printers are widely used as a device adapted to print the character, figure, and the like on print paper by a combination of dots and, known printers of this type include ink jet printers, thermal printers, wire matrix printers, and the like.
A so-called serial type wire matrix printer is the best known and most widely used, with such printer being adapted to print a character or the like on paper set on a platen while moving a wire styli dot matrix print head in parallel to the platen. The wire styli dot matrix head includes seven or nine wire styli arranged in a predetermined manner and guided so that each wire stylus lies in one or two rows at the end portion of the print head. The print head forms dots on a paper by selectively driving electomagnets which correspond in number to the wire styli so that the wire styli are sprung or forced out from the end portion of the print head through respective movable members such as armatures.
Wire styli dot matrix print heads of the aforementioned kind are divided into two types in dependence upon the manner of driving the movable members. One type of print head, described, for example, in U.S. Pat. No. 4,004,673, is called an "attraction type" print head wherein the armatures are attracted by respective electromagnets electrically driven to thereby operate the respective wire styli. The other type of print head described, for example, in U.S. Pat. Nos. 4,044,668 and 4,225,250, are called "cancel type" print heads. More particularly, in the cancel type print heads, a resilient member, having one end thereof fixed, is previously bent by being attracted by a permanent magnet, with the attraction force of the permanent magnet being canceled by the magnetic flux of an electromagnet so that the strain energy stored in the resilient member is applied to the corresponding wire stylis as a printing force.
As compared with the electromagnet attraction type print head, the cancel type print head generates smaller amounts of heat when the printer is on standby and can advantageously ensure a large attraction force by even a small sized permanent magnet. Consequently, the cancel type print head is more suitable for practical use.
In the cancel type print head, a strain energy is stored by bending the resilient member, which energy is employed to drive or fly the corresponding print wire stylis. More specifically, the resilient force of the resilient member is nothing but the printing energy for the printing wire stylis; therefore, the size of the resilient force of the resilient member has a significant effect on the overall printing quality of the print head.
On the other hand, in order to ensure an excellent printing quality in a printer employing a cancel type print head, the resilient force of the resilient member must be regulated so as to enable a realization of an optimum value with respect to variations of each of the magnetic flux amount of permanent magnet and the excitation force of the electromagnet.
Unfortunately, it is exceedingly difficult to regulate and control the amount of resilient force. In this connection, even if the amount of the magnetic flux of each permanent magnet or the excitation force of each electromagnet is constant, it is impossible to obtain uniform forces on the entire resilient members due to the difference in plate thickness between the resilient members or the difference in connection between magnetic members connected to the respective resilient members, that is, a difference in the amount of brazing material used to attach the magnetic member to the respective resilient members. In other words, even if the resilient members are given constant deflection amounts, it will be impossible to obtain a constant strain energy and, consequently, a constant printing quality cannot be obtained for each wire stylis.